The present invention relates to a process for preparing a low-molecular weight chitosan from a high-molecular weight chitosan.
A chitosan is a basic polysaccharide comprising 2-amino-2-deoxy-D-glucoses bonded to each other through .beta.-1,4 glycoside linkages and usually prepared by deacetylating natural chitin. Since the chitosan is basic, it is soluble in an aqueous dilute acid solution. However, since the chitosan is a polymeric compound having a molecular weight of several hundreds of thousands, e.g., a 0.2 % chitosan solution prepared by dissolving chitosan in a solution of 0.8 % sodium acetate in 1.2 % acetic acid has a viscosity (.eta.sp/c) as high as 9 to 16, which makes it very difficult to prepare a chitosan solution having a high concentration.
If it is possible to reduce the molecular weight of the chitosan, a chitosan solution having a high concentration and a low viscosity can be advantageously used in the field of fine chemicals such as pharmaceuticals and cosmetics.
A glucosamine oligosaccharide prepared by reducing the molecular weight of the chitosan has been recognized to have various antibacterial properties and attracted attention as a less toxic antibacterial agent.
Therefore, if the molecular weight of the chitosan can be reduced to a desired range depending upon its application, the chitosan can be further effectively utilized. For this reason, the establishment of a technique for reducing the molecular weight has been eagerly desired in various fields. The following methods (1) to (4) are known in the art for preparing a low-molecular weight chitosan:
(1) a method wherein the glycoside linkages of chitin as a raw material are cleaved to reduce the molecular weight when chitin is deacetylated through alkali treatment.
(2) a method wherein deacetylation is conducted after chitin as a raw material is treated with a dilute acid to cleave glycoside linkages.
(3) a method wherein chitosan is treated with an aqueous hydrogen peroxide solution of an aqueous peroboric acid solution.
(4) a method wherein chitosan is treated with a chlorine gas.
However, in the above-described methods (1) and (2), not only it is necessary to employ severe conditions of heating at 100.degree. C. for 5 to 24 hr but also it is very difficult to prepare chitosan having an arbitrarily reduced molecular weight. Further, since there occurs a side reaction, the resultant chitosan is colored, which renders the chitosan unfavorable for use as a raw material for the above-described fine chemicals.
In the method (3), the glycoside linkages can be cleaved without accompanied by deamination. However, the low-molecular chitosan prepared by this method has a molecular weight of about 10,000 at the lowest and therefore cannot be dissolved in neutral water.
In the method (4), not only a toxic gas must be used but also a colored substance is formed in some cases.
The present invention has been made with a view to eliminating the drawbacks accompanying the above-described conventional methods and based on a finding that cellulase, i.e., a cellulolytic enzyme, invariably has a chitosan decomposing activity, i.e., a chitosanase activity.